Detector system



Feb. 15, 193s. F. G. KELLY, JR 2,108,202

DETECTOR SYS TEM Filed May l2, 1935 2 Sheets-Sheet l y MIIIIIIIII Feb. 15, 1938. F. G. KELLY, JR 2,108,202

DETECTOR SYSTEM Fied May 12, 193s 2 sheets-sheet 2 www Patented Feb. 15,1938

UNITED STATES PATE'' QFF to American Teletector Corporation, New

Haven, Conn., a corporation oi' Connecticut Application May 12, 1933, Serial No. 670,787 6 Claims. (Cl. Z50-27) This invention relates to detector and signalling systems and more particularly to an electrical system for detecting and amplifying any capacity variations occurring in the antenna- '5 ground circuit.

An electrical system embodying the present invention includes `apparatus which shall neglect the slow daily variations in capacity such as those due to changing atmospheric condi- 10 tions, which capacity changes follow the variations in moisture and temperature.

The system may be employed to detect capacity variations caused by foreign bodies entering the antenna eld, by sudden change of l5 temperature, or by sudden changes in the moisture content in the dielectric of said eld or any other object or condition which shall effect the capacity of the system.

More speciically, the invention contemplates a system in which the approach of a body towards a particular point shall cause a change in the electrical capacity of the system and this change is detected by the system. In order to accomplish these results, devices, which are ai- 5 fected by the approach of a body in Such a manner as to change their capacity, must be located at or near places where the approach is to be detected. I call these devices the antenna. While any device having such a charg() acteristic can be used, I will now briefly describe several kinds which I have found suitable in certain applications. If the system is used in window display work so that the display is illuminated when a shopper comes near the 35 window, the antenna may be a fine wire strung across the inside of the window. If a sign is in the Window inviting the shopper to step close and look at the display, a wire screenmay be mounted in back of the sign and used as an- 40 tenna. If the system is used to operate a mechanical door operator, a metal rod may be mounted above the path of approach of the person or vehicle. Ii used as a burglar alarm system, a wir-e or metal rod may be mounted under 45 the window sills or near the doors. All of these are suitable antennas and whichever form of antenna is employed it forms one plate of a condenser and the earth the second plate. lThe surrounding materials, particularly the space oc- 50 cupied by the air, form the dielectric of the condenser, and when a body approaches the antenna field it displaces some of the air and substitutes a dielectric of higher dielectric constant thus increasing the capacity of the con- 55 denser. The rest of the system functions to detect this change in capacity and to indicate it in any desired manner.

In addition to the antenna capacity changes caused by the approach of bodies in the antenna eld, which the system must indicate, there are other changes due to changes in temperature, moisture, weather conditions in general, and also other gradual and seasonal changes. Naturally, these conditions also cause changes in antenna capacity but they should not cause indications. 10 Accordingly, in the practical application of the system it must be able to distinguish between these changes and those caused by the approach of a body. In order to provide for these different changes, I make use of the difference be- 1'5 tween the several types of changes encountered. The seasonal, weather and daily temperature variations cause the antenna capacity to change at a low rate while on the other hand, the approach of a body towards the antenna causes a 0 change at a relatively high rate. I therefore cause the system to indicate capacity changes of a high rate of change but discriminate against those occurring at a low rate. It will be understood that the setting of this limiting rate depends upon the relative values, or constants, of the component parts of the system. In` the following detailed descriptions I have indicated constants and set a limiting rate of change value which I consider suitable but other constants may be chosen and limits may be set without departing from the spirit of the invention.

One important object of the present invention is to provide a detector system and a novel method of operating the same which shall detect the presence of foreign bodies or conditions which cause the antenna-ground capacity to suddenly vary.

Another object of the invention is to provide a system of the character designated which may be adjusted to discriminate against any slow variations in capacity.

A further object of the invention is to provide a system of the character designated which shall be more efficient in operation and able to reject the effects of any slow capacity variations occurring in the antenna field as a result of slow and normally unavoidable changes in the capacity eld and at the same time remain susceptible to more abrupt variations due to the presence oi any foreign bodies.

A further object of the inventionV is to provide a detector system of the character designated embodying a Vnovel arrangement of capacity adjusting devices which may be adjusted so as to render the system sensitive for a wide range of operating requirements.

A further object of the invention is to provide a system of the character designated which may be employed to control the operation of mechanisms associated with door operating devices, safety appliances, material testing and gauging apparatus and similar devices embodying a capacity-sensitive system.

A further object of the invention is to provide a detector system of the character designated in which the discrimination between the rapid and slow capacity variations is a variable factor determined by the time constants of a circuit included in said system.

These and other objects of the invention will be more manifest from the following specification when read in connection with the drawings, and more particularly set forth in the claims.

In the drawings:

Fig. l is a diagrammatic showing of one embodiment of my invention;

Fig. 2 is a diagrammatic showing of a circuit similar to the circuit shown in Fig. l and is designed for handling large antenna capacities, and

Fig. 3 shows the resonator' voltage curves of the embodiment shown in Fig. 1.

Referring to the drawings there is shown in Figs. 1 and 2 circuits embodying-the general principles of my invention and it is understood that certain circuit modifications may be made in order to provide for the most emcient operation under various operating conditions. For example, the circuit shown in Fig. l is particularly desirable where the capacity of the antenna system is relatively small and where conditions are such that changes in antenna capacity due to temperature, humidity, weather conditions in general and mechanical shifting of the antenna, etc., are small.

The circuit shown in Fig. 2 however is suitable where the antenna capacity is large and where the gradual changes in the antenna capacity are large.

In both of the systems designated above, the antenna is connected through a buffer condenser to an oscillator which may be of any suitable type, although the oscillators as shown are of the plateV circuit type. The oscillator furnishes an alternating output voltage varying in frequency according to changes in the antenna circuit capacity. This oscillator output is fed to a tuned resonating circuit which provides Wide voltage variations in response to shifting frequencies. The output of this resonating circuit is rectified and then amplified, while all changes in the rectied voltage are amplied, only rapid changes are amplified efficiently enough to affect the succeeding circuits. The output circuit of the amplier, for example, contains a relay which is actuated in response to these voltage surges, and which is used to place in operation any desired signalling, operating or indicating devices such as those heretofore mentioned.

summarizing, it is apparent that sudden changes in antenna capacity manifest themselves in a change of oscillator frequency, causing a large change in both the resonator voltage and the rectified resonator voltage. Changes in the rectified voltage are then amplified and actuate suitable relay contacts which xnay be connected in any outside circuit and thus effect the desired control of associated mechanisms for which the system may be designed.

Referring to Fig, 1 in more detail, the values given provide a system in which the antenna capacity may go up to mmf. and in which the expected gradual changes in antenna capacity will not exceed plus or minus 1 mmf. The abbreviation mmf. will be used to indicate micromicrofarads and mf. to indicate microfarads.

Any suitable antenna system is connected to the antenna terminal I and a good connection to ground is made at terminal 2. A buffer condenser 3 is interposed between the antenna terminal and an oscillator tuning circuit il. One side of the oscillator tuning circuit leads to a source d3 of 45 volts positive, while the other side leads to the plate 5 of a vacuum tube 6. The oscillator circuit 4 is tuned by an inductance 'i of two millihenrys value and a variable condenser 8 of 100 mmf., connected in parallel. A control grid 9 of tube 6 is connected through a feedback coil IQ which is inductively coupled to coil l, to a parallel circuit consisting of a condenser ii of 1GO mmf. and a resistance I2 of a half megohm. The other side of the condenser I I and resistance I2 is grounded. This method of connecting the grid maintains a negative grid bias at the least value which will keep the grid from swinging appreciably positive and results in a particularly stable oscillator. The values chosen provide a minimum of grid circuit inductance and at the same time avoid relaxation, periodic starting `and stopping of oscillation. The cathode element I3 of tube 6 is grounded and an associated heater element lll is connected to a suitable source of current supply 45 which is indicated as a battery.

Figure 3 shows two curves of voltage across the resonator plotted against oscillator capacity. These curves were obtained experimentally from actual systems. It will be noted that the points of resonance diifer slightly due to slight variations in the lcircuit constants and tube characteristics, but if each system is adjusted to an oscillator capacity of l mmf. greater than the resonance capacity, the slope of the voltage curve for a variation in antenna capacity of l mmf. either side of this setting is the same. This portion of the curve may be called the range of the system.

The alternating output voltage of the oscillator fed through a condenser it of 4 mmf. to a parallel resonating circuit consisting of an inductance ll of .75 millihenry and a condenser i8 of 250 mmf. These values result a resonance point of approximately 371,000 cycles, at which frequency the voltage across the inductance Il is at a maximum, and any deviation from this frequency will lessen this/voltage. -Ccnnected to this resonating circuit is a further parallel circuit including a condenser I9 of .1 mf. and a resistance 2B of one megohm.

The tube 2| has its control grid 22 connected through a grid coupling condenser 23 of two mf. capacity to a point 24 between condensers I3 and i9. When the system is shut off the resistance leak 2 discharges condenser IS so that the point 24 is at ground potential. l'f the ground is considered the reference line, when the system is turned on, the oscillations, occurring with point 24 as reference point, will cause a rectifier plate 25 of tube 2| to swing alternately positive and negative. When rectifier plate 25 is positive with respect to the cathode 26 of tube 2l, it draws electrons from it. These electrons accumulate in condenser I9, causing point 24 to become negative with respect to the cathode 2t. This accumulation continues until the negative potential of point 24 is approximately equal to the peak potential across resonator coil I1. At that time, the rectifier plate 25 no longer swings appreciably positive and point 24 remains at a constant negative potential. lIhis negative potential equals about 90% of the resonator peak Voltage because the resistance leak 28 continues to discharge condenser |9 and so some slight replacement current must come from rectifier plate 25 in order to balance the loss.

When the peak resonator potential drops in response to the system detecting the presence of a foreign body, it does so abruptly, and the resistance leak 20 immediately drains the opposing negative potential down to the new resonator peak Voltage. The `discrimination between seasonal Variations and alarm variations is as follows:

The grid 22 of tube 2| is connected to point 24 through the coupling condenser 23 of 2 mf. This condenser is relatively large and any change in its potential can occur only by current flowing through the high resistance 21 of 3 megohms, which current flow requires an appreciable length of time, thus establishing a large time constant in this portion of the circuit. Any sudden change in potential of point 24 can cause condenser 23 to readjust itself to this change only after a long time interval, while a gradual change due to the eiect of seasonal variations is able to leak through resistance 21 without appreciably altering the balance of potentials. Until this adjustment of potential in condenser 23 is accomplished, the potential of grid 22 is less negative causing an increase in output current of the tube 2|. A negative bias of two volts from any suitable source 44 is applied to the grid .22 through resistance 21. A heater element 28 of tube 2| is energized by a suitable power source 45.

The plate current of tube 2| flows through a plate 29, the resistance 30 of 50,000 ohms to a 90 volt source 43, a battery in this instance. A point 3| between plate 29 and resistance 38, has a potential of about 25 volts due to the total resistance of the circuit. Point 3| is coupled to a control grid 33 of a vacuum tube 38 by means of a coupling condenser 32 of 4 mf. The grid 33 of tube 38 is connected to the cathode 36 through a resistance 34 of 3 megohms having a bypass condenser 35 of .01 mf. shunted across it. A heater element 31 of tube 38 is supplied with current from the common heater source 45.

The following action takes place: When equilibrium is reached the difference between the potential of the grid 33 and point 3| appears as a potential across condenser 32. An increase in plate current of the preceding tube 2| causes a drop in potential of the point 3l. The condenser 32 cannot adjust itself instantly as the charge must leak through the high resistance 34, and there follows a drop in the potential of grid 33 resulting in a reduced output current of tube 38.

The plate 38 of tube 38 includes in its circuit a relay coil 40 and a voltage source 42. Sufficient current is normally flowing in this circuit to hold relay contacts 4| open, but when the current suddenly drops in response to a functioning of the antenna capacity detecting circuits, the relay is de-energized and its contacts drop closed, thus completing the outside circuit of an alarm or operating device. The de-energizing of relay 48 is caused by a change of antenna capacity of .1 mmf. occurring in 1/4 second or less.

The system, as illustrated in Fig. 1 is suitable for such applications as Window displays, door operating, etc. in other words, where the antenna capacity is comparatively small and where the seasonal variations in antenna capacity are small.

InV other applications,V such as burglar alarm work, when all points of entry to a home may have to be covered, a larger system handling a much greater amount of antenna capacity, but causing indications for the same absolute change in antenna capacity, is needed. One such modiflcation of my invention is shown in Fig. 2.

In this embodiment, the system is designed to carry an antenna of 1000 mmf. capacity and able to adjust for seasonal changes up to plus or minus 200 mmf. and still maintain approximately uniform sensitivity over the entire range. This system will give an indication for an antenna capacity change of .1 mmf. occurring in 1/4 second or less.

The system as shown in Fig. 2 employs the same general principle of operation as the system shown in Fig. 1, however the Values of components differ in order to accommodate the different antenna capacity conditions. In this system, an antenna terminal 5| and a ground terminal` 52 are provided and a condenser 53 of .1 mf. capacity is connected between the antenna terminal and an oscillator portion of this system which is of the same type as that shown in Fig. 1. A vacuum tube 54 is provided with an oscillator coil 55 and a variable tuning condenser 56 of 1000 mmf. capacity furnishing a frequency of 200,000 cycles. An oscillator feedback coil 51 is inductively coupled to coil 55, and is connected to a control grid 58 of tube 54. The other side of the feedback coil 51 leads to a parallel biasing circuit composed of a resistance 60 of 200,000 ohms and a condenser 59 of 250 mmf. capacity. The other side of the biasing circuit is grounded. A bypass condenser 6| keeps large circulating currents out of the remaining portion of the system.

The oscillator voltage is applied through a condenser 62 to a series resonator comprising a coil B3 of 10.4 mh. inductance and a variable condenser 84 of 50 mmf. capacity. A series resonant circuit is employed in this instance because the values of its component parts are more practical than those of a parallel resonant circuit under the given conditions and desired operating characteristics. Furthermore, in a series resonant circuit changes in supply voltage are less detrimental because the resonator voltage is a smaller percentage of the supply voltage than in the parallel type.

The series resonator is tuned to 220,000 cycles and the voltage across coil 63 varies from zero atresonance, to about 10% of the oscillator voltage at 180,000 cycles, or about one volt. Resistance 65 of 50,000 ohms, and a parallel coil and condenser circuit 66 comprise a trap circuit which removes the larger portion of oscillator harmonics. The tube 61 is a radio frequency amplifier stage which is connected in the conventional manner as shown in Fig. 2 and amplies the output voltage of the resonator circuit.

The amplified resonator voltage is led through a condenser 68 to a resistance 1| connected to a rectifier plate 12 of the tube 13. Electrons flow from the cathode of tube 13 to the rectifier plate 12, through the resistance 1| and coil 10, to condenser 14 of .5 mf. value. This negative accumulation occurs only when the resonator voltage swings positive and continues until the negative potential is substantially equal to the peak of the amplified resonator voltage. The high resistance leak 15 of 10 megohms causes a slight loss of potential in the circuit, and rectifier plate 12 swings positive just enough to balance the loss through leak 15.

If a body approaches the antenna, causing an increase in antenna capacity, the oscillator frequency decreases causing an increase in resonator voltage. This voltage is amplified by the tube 61 and causes the rectifier plate 12 to swing more positive, increasing the negative charge on the condenser 14. The constants of the circuit are such that the adjustment of the charge on condenser 14 occurs almost instantly. The approach of the body, therefore, causes an immediate increase in the negative potential of condenser 14.

'I'he difference in voltage between the grid 16 of tube 1 3 and the condenser 14 appears as a potential across condenser 19. The sudden increase in negative potential of condenser 14, caused by the approach of the body, results in a decrease in potential of the grid 16 since condenser 19 cannot instantly readjust its charge. The grid leak 18 must carry the current necessary to change potential on condenser 19, and the high resistance of leak 18 slows down the adjustment so that a suitable time lag is provided. The grid 16 being more negative during this interval causes a reduced plate current in tube 13. The potential of plate 80 of tube 13 rises due to its reduced plate current. A condenser 84 is connected between plate 80 and the control grid 82 of a tube 83. The rise in potential of plate 30 causes an increase in potential of grid 82 since condenser 84 cannot adjust its charge instantly through the delay action of grid leak 85, therefore an increased plate current with its consequent drop in plate potential occurs in tube 83 as a result. This decrease in plate potential is impressed upon a condenser 9| and causes the grid 89 of tube 90 to become momentarily more negative, and the plate current to decrease. A relay coil |03 in the output circuit of tube 90 is de-energized in response to the suddenly reduced plate current, and allows contact arm |04 to drop closed which completes an outside alarm or operating circuit.

Batteries have been shown as sources of energy supply, however a rectied alternating current may be used provided the direct current output is regulated so that the direct current voltage does not vary at a rate greater than the system can provide for.

A sudden drop in the voltage of the oscillator supply reduces the resonator voltage in precisely the same way it would be reduced by a sudden increase in antenna capacity. In other Words, the system cannot distinguish between increase in antenna capacity and decrease in the supply voltage. Accordingly, either batteries or regulated rectied alternating current should be used, the regulation being such that changes in the alternating supply voltage, regardless of the rate at which they occur, will be slowed down so that the corresponding change in the direct current voltage will occur at a rate which is slower than the minimum rate at which the syst-em will function. l i

For the purpose of illustrating a practical embodiment of the apparatus shown in Fig. 1, the outside circuit contacts 4| are arranged to control a door |05. 'Ihe Adoor operating device is shown diagrammatically by a motor |06 and lever connections |01. The motor |06 is connected in circuit with contacts 4| and a source of current |08 indicated as a battery. The antenna for the door is indicated by a pair of suitable metallic plates |0S, |09 placed on opposite sides of the door and connected to the antenna terminal by leads ||0, H0. A suitable ground connection or counterpoise may be in close proximity to the door.

Should an object or person approach the door from either side and in close proximity to the antenna, the capacity of the system is aifected as heretofore described, to close the contacts 4| and energize the motor |06 to operate the door. Referring to the embodiment shown in Fig. 2, a window indicated by the numeral ||2 is to be protected by a burglar alarm indicated by a bell ||3 connected in an outside circuit with contacts |04. In this system, the Window is provided with an antenna wire ||4 which may be placed adjacent thereto in any desired manner, concealed in the easement, for example. A ground or counterpoise connection indicated by numeral ||5 may be placed in close proximity to the window. The bell H3 is connected in circuit with the contacts 04 and a suitable source of current indicated by a battery I0. It will thus be noted that should any intruder or unauthorized person approach the window, the capacity of the system will be affected and close the contacts |04 as heretofore described, and energize the bell circuit.

While I have shown an antenna connection for only one windcw, it is obvious that all the windows and doors of a building or dwelling may be provided with similar antenna connections and thereby provide a reliable and eflicient burglar alarm.

While I have shown and described the preferred embodiments of my invention it will be obvious that modi cations and changes may be made therein without departing from the scope of the invention as defined in the following claims.

What I claim is:

1. In a vacuum tube apparatus, an antenna having a capacity to ground, means for discriminating against seasonal changes in said antenna-ground capacity comprising an oscillator connected to said antenna and having its frequency of oscillation variable in response to changes in antenna-ground capacity, means for establishing a source of direct current potential which varies in value with variations in oscillator frequency, a coupling circuit comprising a condenser and a resistance connected to said source and having a time constant smaller than the time duration of said seasonal changes in antenna-ground capacity, an amplier connected to said coupling circuit, and translating means connected to said amplifier and responsive to the output thereof.

2. In a vacuum` tube apparatus, an antenna having a capacity to ground, means for discriminating against seasonal changes in said antenna-ground capacity comprising an oscillator connected to said antenna. and having its frequency of oscillation variable in response to changes in antenna-ground capacity, means including a rectier for establishing a source of direct current potential which varies in value with variations in oscillator frequency, a coupling circuit comprising a condenser and a resistance connected to said source and having a time constant smaller than the time duration of said seasonal changes in antenna-ground capacity, an amplifier connected to said coupling circuit, and translating means connected to said amplier and responsive to the output thereof.

3. In a vacuum tube apparatus, an antenna having a capacity to ground, means for discriminating against seasonal changes in said antennaground capacity comprising an oscillator connected to said antenna and having its frequency of oscillation variable in response to changes in antenna-ground capacity, a resonating circuit connected to the output of said oscillator, means for establishing a source of direct current potential which varies in value With changes of the oscillating potential developed across said resonating circuit, a coupling circuit comprising a condenser and a resistance connected to said source and having a time constant smaller than the time duration of said seasonal changes in antenna-ground capacity, an amplifier connected to said coupling circuit, and translating means connected to said amplier and responsive to the output thereof.

4. In a vacuum tube apparatus, an antenna having a capacity to ground, means for discriminating against .seasonal changes in said antenna-ground capacity comprising an oscillatoi` connected to said antenna and having its frequency of oscillation Variable in response to changes in antenna-ground capacity, a resonating circuit connected to the output of said oscillator, means tuning said resonating circuit to the normal frequency of said oscillator, means for establishing a source of direct current potential which varies in Value with changes of the oscillating potential developed across said resonating circuit, a coupling circuit comprising a condenser and a resistance connected lto said source and having a time constant smaller than the time duration of said seasonal changes in antennaground capacity, an amplier connected to said coupling circuit, and translating means connected to said amplifier and responsive to the output thereof. Y

5. In a vacuum tube apparatus, an antenna having a capacity to ground, means for discriminating against seasonal changes in said antennaground capacity comprising an oscillator connected to said antenna and having its frequency of oscillation variable in response to changes in antenna-ground capacity, a resonating circuit connected to the output of said oscillator, means tuning said resonating circuit to the normal frequency of said oscillator, a rectifier connected to said resonating circuit, means in said rectifier circuit for establishing a steady direct current potential which varies in value With Variations in the oscillating potential across said resonating circuit, a coupling circuit comprising a condenser and a resistance connected to said source and having a time constant smaller than the timev duration of said seasonal changes in antennaground capacity, an amplifier connected to said corupling circuit, and translating means connected tosaid amplifier and responsive to the output thereof.

6. In vacuum tube apparatus, an antenna, an oscillator connected with said antenna, means including said antenna for regulating the frequency of said oscillator, a resonating circuit connected to said oscillator and energized by the output thereof, means tuning said resonating circuit to substantially the normal frequency of said oscillator, a rectifier connected to said resonating circuit, resistance means connected in circuit With said rectier whereby -a steady direct current potential is established in said rectifier circuit, a coupling condenser having a terminal connected to one end of said resistance means, an amplifier having an input circuit connected to the other terminal of said coupling condenser, a second resistance connected in the input circuit of said amplifier, and translating means responsive to the output of said amplier.

FREDERICK G. KELLY, JR. 

